Add isAdmin() and exceptions to UserManager

Details

1. UserManager can be in read-only mode anf throw UnsupportedIperationException, if it doesn't support creating new users or updating existing
2. UserManager can contains multiple admin account. So, default admin name account need only at first logon to autocreate admin.

1. UserManager can be in read-only mode anf throw UnsupportedIperationException, if it doesn't support creating new users or updating existing
2. UserManager can contains multiple admin account. So, default admin name account need only at first logon to autocreate admin.